The invention relates to a method of drying and preheating moist fine material, such as pit coal and lignite, peat, wood, oil shale, as a preliminary stage to carbonization, or even ores, or limestone for the cement production, in which the fine material is fed to a drier-heater system and moved therein along heating devices and then removed from the drier-heater system. The invention further relates to an apparatus for carrying out the method.
Moist fine material may be dried and preheated either directly by means of hot inert or flue gases, for example, in conventional flash driers, or by means of indirectly heated surfaces or tubes. In the indirect method, it is necessary to obtain a satisfactory heat transfer with an extended surface and space utilization and a minimum layer thickness, without the material which is to be dried, congesting or forming bridges. Several methods have been provided in this regard.
A combined direct and indirect drying and preheating is known from the fluidized bed drying. A hot fluidizing gas is blown in the fine material charge, to improve the heat transfer and obtain a satisfactory fluidity, or to avoid congestions, in addition to the indirect heat transfer through heating coils.
Both in flash drying and the fluidized bed drying, a considerable amount of electric energy must be supplied to circulate the hot gases. In addition, a large dust separating equipment is needed.
The drying and preheating fine coal by means of heating tube nests vibrating at natural frequency in a conveyor trough is also known. Since the natural frequency depends on the size of the tubes, narrow limits are set for drier performance. In addition, the tube material stresses and the electric energy requirements are high.
Finally, so-called rotary drum driers are known which are equipped, in their interior, with heating tube nests as well as with means for feeding and discharging the moist and preheated fine coal. The rotary drive itself provides the transverse and vertical displacement of the charge. To this end, the drum is mounted with a slight inclination and provided with inserts by which the coal fed to, and accumulating at, the bottom of the drum is raised above the heating tube nests arranged at the center of the drum, and dropped on the tube nests. Due to this slight inclination of the rotary drum, the fine coal fed in at one end migrates to the other end where it is removed. During this travel, the coal is repeatedly caused to fall on the tube nests where it dries while sliding down on the tubes freely, without piling up.
To prevent congestions and formation of bridges at the entrance, certain minimum fluidity requirements must be imposed on the charged material.
The operation of such rotary drums having a stationary internal equipment raises particular problems with the gas-tight sealing of the drum relative to the stationary heads in which the tube nests are mounted.